The present invention relates generally to wireless communication devices and in particular to a linear power amplification system of a wireless communication device.
Power amplifiers for wireless transmission applications, such as radio frequency (RF) power amplifiers, are utilized in a wide variety of communications and other electronic applications. Ideally, the input-output transfer function of a power amplifier should be linear, with a perfect replica of the input signal, increased in amplitude, appearing at the output of the power amplifier.
In addition, for greater efficiency, various RF systems, such as cellular systems, attempt to run power amplifiers at or near their saturation levels, in which the actual output power of the amplifier is just below its maximum rated power output level. This power output level is generally related to the supply voltage (or supply power) to the power amplifier, such that a greater supply voltage will produce a correspondingly greater output power from the amplifier; for higher power input signals, a correspondingly greater actual power output is required to maintain the amplifier at or near saturation. In various prior art amplifiers, however, the supply voltage to the power amplifier is fixed. Given a typical usage situation in which actual power output from the amplifier may vary by a range of several orders of magnitude, use of a fixed supply voltage is highly inefficient, as output power is often an order of magnitude below its maximum, and the power amplifier is not maintained at or near its saturation levels.
Various techniques have evolved to vary the supply voltage to maintain the power amplifier at or near saturation. One such technique is power supply modulation (PSM) which varies, or modulates, the supply voltage to the power amplifier in order to maintain the power amplifier at or near saturation while the input signal varies over time. For PSM, the supply voltage of the amplifier tracks the input signal variations, typically utilizing a signal detector in conjunction with a tracking power supply. In the prior art, however, the various PSM techniques have generally been limited to narrowband applications, or have poor efficiency characteristics.
For example, one prior art PSM technique, known as xe2x80x9cenvelope elimination and restorationxe2x80x9d (EER), utilizes a limiter to provide an essentially constant drive level to the power amplifier to maintain the amplifier in a hard saturation state and increase efficiency. Use of the limiter, however, greatly expands the bandwidth of the RF signal input to the amplifier and requires very accurate tracking of the input signal envelope, with a power supply switching frequency approximately ten times greater than the bandwidth of the RF input signal. As these switching frequencies increase, the transistors within the tracking power supply become less efficient, resulting in excessive power losses. The resulting bandwidth expansion of the limiter also requires the bandwidth capability of the amplifier to be significantly greater than the input signal bandwidth, limiting the EER configuration to narrow bandwidth applications, such as amplitude modulation (AM) RF broadcasts.
Another prior art PSM technique, known as xe2x80x9cenvelope tracking,xe2x80x9d does not utilize the limiter of EER and consequently may be suitable for higher bandwidth applications. Use of envelope tracking, however, introduces significant non-linearities in the output signal of the power amplifier, such as gain distortions, phase distortions, and other voltage parasitics. These non-linearities result in an introduction of an additional magnitude component and phase component to the original signal. If these distortion characteristics are not compensated they will cause intermodulation distortion (xe2x80x9cIMDxe2x80x9d) in multicarrier frequency division multiple access (xe2x80x9cFDMAxe2x80x9d) or time division multiple access (xe2x80x9cTDMAxe2x80x9d) systems, and spectral growth in code division multiple access (xe2x80x9cCDMAxe2x80x9d) systems. The various distortions degrade output signal quality and may have other detrimental effects, such as decreased data throughput.
In order to counteract the distortion introduced to an input signal by a power amplifier in an envelope tracking system, techniques have been developed for injecting a predistortion signal into an input signal""s path prior to amplification. The predistortion signal includes components equal and opposite to the distortion introduced by the power amplifier and are designed to cancel the distortion introduced to the input signal by the power amplifier. For example, FIG. 1 is a block diagram of a linear power amplification system 100 of the prior art. The operation of a linear power amplification system such as system 100 is described in detail in U.S. patent application Ser. No. 09/765,747, entitled xe2x80x9cHigh Efficiency Wideband Linear Wireless Power Amplifier,xe2x80x9d which application is assigned to the assignee of the present invention and is hereby incorporated by reference herein in its entirety. System 100 includes an envelope detector 102, a tracking power supply 104 coupled to the envelope detector, an input signal conditioner apparatus 120, and a power amplifier (PA) 110. Power amplifier 110 is preferably coupled to an antenna (or antenna array) 114 for wireless transmission of an amplified, output signal 112. System 100 also includes first and second delay circuits 106 and 108, respectively.
Envelope detector 102 and tracking power supply 104 are utilized to track a signal 101 input into system 100 and to provide a variable supply voltage 109 to the power amplifier 110. Variable supply voltage 109 is designed to maintain power amplifier 110 at or near saturation and to increase the efficiency of the power amplifier over a wide range of variation of the input signal. However, the variation of the supply voltage 109 supplied to power amplifier 110 causes gain and phase distortions to be introduced to the amplified signal 112 by power amplifier 110.
In order to counteract the gain and phase distortions introduced by power amplifier 110, input signal conditioner apparatus 120 predistorts, or conditions, input signal 101. Input signal conditioner apparatus 120 includes a nonlinear phase mapper 122, a phase adjuster 124, a nonlinear gain mapper 126, and a gain adjuster 128. Variations in gain and phase in the output signal 112 are correlated to, or otherwise occur as, a function of the supply voltage 109 sourced to power amplifier 110. The variations in gain and phase of output signal 112 can be calibrated or otherwise empirically determined as a function of supply voltage 105, 109 to create nonlinear phase and gain mappings that are respectively implemented in nonlinear phase mapper 122 and nonlinear gain mapper 126.
Non-linear phase mapper 122 and non-linear gain mapper 126 are then used to adjust the transfer functions of phase adjuster 124 and gain adjuster 128, respectively, based on the supply voltage 109 sourced to power amplifier 110. By adjusting the transfer functions of phase adjuster 124 and gain adjuster 128, the phase and gain of input signal 101 can be adjusted in such a manner as to counteract the gain and phase distortions introduced to the input signal at varying supply voltages by power amplifier 110.
A problem with a linear power amplification system such as system 100 is that the system provides optimal performance only under the environmental conditions, such as power amplifier 110 age and operating temperatures, at which non-linear mappers 122 and 126 are set up. It is well known in the art that power amplifier performance varies with both an age and an operating temperature of the amplifier. Therefore, there is a need for a linear power amplification system that is more inherently self-aligning, that is, that self-corrects for variations in power amplifier 110 performance due to the effects of age and operating temperature on a power amplifier.